Electroforming technique and structure for reflecting mirrors



Apr-11 16, 1968 P. R. JOCHIM 3,378,469

ELEGTROFORMING TECHNIQUE AND STRUCTURE FOR REFLECTING MIRRORS FiledApril 5, 1964 INVENTOR. PER Q) K EXJOCH/M A 77'0RA/Ey United StatesPatent Ofice 3,378,469 ELECTRGFORMING' TECHNIQUE AND STRIK- TURE FORREFLECTING MIRRURS Perry Rex .loehirn, Temple (Iity, Calih, assignor toElectra-Optical Systems, Inc., Pasadena, Caiif. Filed Apr. 3, 1964, Ser.No. 357,170 7 Claims. (Cl. 204-7) The present invention relates ingeneral to the electroforming arts and more paritcularly relates to theprovi sions of a backing or rigidizing structure for an electroformedproduct.

In electroforming various kinds of devices, such as mirrors, it isnecessary to provide a backing structure for these devices, primarilybecause the electroformed metal layer is thin and Will buckle orotherwise distort unless properly supported. In providing such a supportstructure, it has been generally necessary in the past to go through anumber of steps which were not only time consuming and not onlyincreased the cost of fabrication but, very importantly, because of theadditional handling, exposed the thin electroformed surface to thedanger of damage.

More specifically, in fabricating a device by means of theelectroforming process, such as a parabolic reflecting mirror, forexample, a layer of release material, such as copper, is vacuumdeposited on the concave surface of a parabolic glass master. A layer ofsilicon monoxide is then vacuum deposited over the release layer,the'silicon monoxide being used to provide a hard surface. After this,an epoxy resin mixture is poured in over the silicon monoxide layer andcured while the master is rotated at a uniform speed. By so doing, theepoxy material spreads outward because of the centrifugal forcesinvolved and thereby assumes the shape of the glass master. The epoxycures in this parabolic shape and, together with the silicon monoxidelayer beneath it, forms what is known as the sub-master. A backingstructure, such'as a stiff plate, is

then mounted onto the sub-master for support and thereafter separatedfrom the glass master itself.

At this point, chemical silver is sprayed onto the working surface ofthe sub-master and the sub-master is then mounted as the cathode on arotating spindle, the entire combination being immersed in a platingbath or tank wherein, as the spindle is rotated, a uniform layer ofnickel is plated over the silver. When the nickel is at the rightthickness, the structure is removed from the tank and a backing orrigidizing structure mounted upon the nickel layer to provide it withthe necessary support. The nickel layer, together with its backingstructure, constitutes the electroformed mirror and, at this point, itis physically separated from the sub-master structure.

As has already been mentioned, a major disadvantage of this technique isthat a rigidizing structure for the electroformed mirror is mounted onlyafter the nickel layer, which provides the reflecting surface for themirror, is fully formed. As a result, the nickel layer and, therefore,the mirror itself, is exposed to the danger of being damaged which, inturn, makes the entire electroforming process a highly vulnerable one.Accordingly, there has existed a long-felt need for a comparatively safemethod for providing these backing structures. The present inventionfulfills this need.

More particularly, according to the basic concept of the invention, thebacking or rigidizing structure is also electroformed and, therefore,can be provided simultaneously with the electroformed device itself,whether it be a mirror or some other device. As will be explained ingreater detail later, this is done by halting the electroplating of thedevice before it is finished and applying a properly shaped mesh to it.The electroplating is then continued so that the mesh becomespermanently affixed to and becomes an integral part of the device beingelectro- U 3,378,419 Patented Apr. 15, 1968 formed, thereby providingthe backing structure for it in substantially the same step in which thedevice is formed.

Accordingly, it is an object of the present invention to provide a newplating technique by means of hich an electroformed surface and thebacking structure for it are fabricated at the same time.

It is another object of the present invention to provide anelectroforming technique which very greatly reduces the possibility ofthe device being electroformed from being prematurely damaged.

It is a further object of the present invention to provide a techniqueby means of which a backing structure is provided for a thinelectroformed layer with minimum risk of damage to the layer.

The novel features which are believed to be characteristic of theinvention, both as to its organization and method of operation, togetherwith further objects and advantages thereof, will be better understoodfrom the following description considered in connection with theaccompanying drawing in which an embodiment of the invention isillustrated by way of example. It is to be expressly understood,however, that the drawing is for the purpose of illustration anddescription only and is not intended as a definition of the limits ofthe invention.

FIGURE 1 is a front view, in cross-section, of a glass master and thesteps involved in the formation of a plastic sub-master from it;

FIGURE 2 is a front view, in cross-section, of the plastic sub-mastermaintained in its shape by a rigidizing structure;

FIGURE 3 is the FIG. 2 sub-master but with its outermost layer, namely,the release layer, removed;

FIGURE 4 is a repeat of the FIG. 3 sub-master but with an additionalouter conductive or sensitizing layer coated on it;

FIGURE 5 is a front view, in cross-section, of a mesh backing structureafter it has been affixed to the surface being electroformed;

FIGURE 5(a) is a perspective view of the FIG. 5 structure;

FIGURE 6 is a front view, in cross-section, of the mesh backingstructure after it has become permanently mounted to the surface beingelectrofor med;

FIGURE 6(a) is a perspective view of the FIG. 6 structure; and

FIGURE 7 is a perspective view of the final electroforrned mirrorproduct.

In considering the invention in detail, it should be emphasized andrecognized at the outset that although the mounting and fabrication of abacking structure for aparabolic mirror will be described below as wellas the process by which it is made, the process is not limited to thefabrication of mirrors alone but, rather, is applicable to thefabrication and rigidizing of all other kinds of electroformed devicesor products.

With this in mind, reference is now made to FIG. 1 wherein is shown aparaboli-cally-shaped glass master 10 that may be used in thefabrication of the electroformed parabolic mirrors. In practicing theinvention, the glass master is first mounted on a suitable support (notshown) that holds the master in an undistorted position during the firststeps of the process. Because such mounts or supports are so well knownand are conventional in the electroforming art and, furthermore becausethey are not a part of the present invention, a mount or support is notshown herein.

Having mounted the glass master for the reason mentioned, the opticalsurface area of the master is then thoroughly cleansed by any good glasscleaning technique that is known and employed by the optical coatingindustry. Following this, a non-adherent and pin hole free metal releaselayer 11 is vacuum deposited on the optical surface of the glass master.A number of different kinds of metal may be used as the release layer,such as copper, which is the preferred metal, silver, gold, etc., theonly criterion being that it should be thick enough to be opaque but nottoo thick or else its surface will become granular, a. condition that ispreferably avoided. Vacuum chambers and vacuum deposition are well knownin the arts and sciences and, therefore, it is not deemed necessary hereto describe a vacuum chamber in detail or the manner in which it isused. If a hard protective surface coating or other special surfacecoating is required over release layer 11, it may be also vacuumdeposited at this time. Thus, in FIG. 1, a hard protective layer 12,namely, a layer of silicon monoxide, is shown coated over release layer11.

At this point, the coated master is removed from the vacuum chamber andfirmly mounted either on a spin fixture or on a casting fixture of somekind, the particular fixture employed being dependent upon therequirement for a parabolic or other shape. Since a parabolic figure isinvolved in the present instance or description, the coated glass masteris mounted upon a spin fixture of the kind disclosed by Burt J. Bittnerin the patent that issued Nov. 28, 1961, Patent No. 3,010,153, for hisinvention entitled Construction of Paraboloid Surfaces. With the mastermounted on the spin fixture, a plastic, such as an epoxy resin mixture,is then poured in over silicon monoxide layer 12, the fixture then beingactivated to rotate at a uniform speed. By so doing, the plasticmaterial spreads outward because of the centrifugal forces involved,with the result that it assumes the parabolic shape of the glass master.The plastic is cured as it rotates to ultimately produce a plastic layerhaving substantiall the same shape and optical accuracy as the glassmaster itself. The plastic layer spoken of above is desig nated 13.

Having reached the stage shown in FIG. 1, the next step in the processis that of parting or separating the plastic submaster from the glassmaster. However, prior to this step, the plastic submaster is rigidizedwith a suitable stable structure, such as a plate or weldment, etc.,which is fixed to the plastic layer at its edge. Referring now to FIG.2, the abovesaid stable structure is designated 14 therein and is fixedto a plastic layer 13 by means of an epoxy or other adhesive 15interposed between the plastic layer and the rigidizing structure, as isshown in the figure. With member 14 properly mounted, glass master 10 isremoved to leave the remaining structure shown in FIG. 2. Release layer11 having now served its purpose, it is removed with nitric acid orother chemical materials to leave the plastic submaster which, in FIG.3, is shown to comprise plastic layer 13 and its hard optical surface 12made of silicon monoxide or some equivalent material.

With the completion of the plastic submaster, the next major part of theprocess is initiated, namely, that of elec troforming the desiredparabolic mirror. To do so, the plastic submaster is immersed in anelectroplating bath but, before it is immersed, it must be properlyprepared for it. Accordingly, the optical surface of the submaster isfirst cleaned in the same well-known manner as was the glass masterduring the first steps of the process and then it is sensitized orrendered electrically conducting by coating it with a thin film or layerof electrically conductive material, such as chemical silver, which iscustomarily used for this purpose. In the event that silver is used, itmay be sprayed on the optical surface as is done in commercial mirrorfabrication. A thin film or layer of silver is shown deposited on theoptical sunface of the plastic submaster in FIG. 4 and is designated 16therein. At this point, the submaster is ready for the electroplatingbath and, therefore, it is mounted as the cathode and thereafterimmersed in the electroplating solution.

Although any one of a number of different kinds of plating solutions maybe utilized, as is well known by those skilled in the electroplatingart, a nickel plating solution is preferred in the present instance.Accordingly, employing necessary controls to insure a uniform stressfreedeposition, a layer of nickel 17 is electroplated over silver layer 16.What is meant by necessary controls is already known in the art. Sufficeit to say, therefore, that it involves proper circulation andtemperature maintenance of the electroplating solution, uniform rotationof the submaster cathode, maintaining a uniform composition or strengthof the electroplating solution, etc., all of which contribute to thedeposition of a nickel layer of uniform thickness and that is relativelystress-free.

When the nickel la3 er is still but a fraction of the desired thickness,the submaster is removed from the bath for the purpose of applying amesh structure to it that will ultimately become a rigid backingstructure that will support the nickel layer so that it wont buckle,bend or otherwise distort. Thus, following the removal of the submaster,a mesh backing structure 13 is mounted over nickel layer 17, the backingstructure being of such shape and dimension that it makes contact withthe nickel layer along the edge of the latter. However, at this stage,the backing structure need not be and, therefore, is not, permanentlymounted or fixed to the nickel. Rather, it is only necessary that thebacking structure make good contact with the nickel all along its edgeand remain in good contact with it when the electroplating sequence isresumed. Accordingly, it is only necessary to apply a temporary holdingmaterial or substance to the mirror elements whereat they are in contactwith each other, such as a wax coating, a suitable paint, or the like.

A backing structure thusly held in position on the nickel layer is shownin cross-section in FIG. 5, a perspective view of it being illustratedin FIG. 5(a). As was previously indicated, the nickel layer isdesignated 17 and the backing structure 18, the substance applied tohold the two of them together being designated 20.

With respect to the kind of materials that may be used for the backingstructure, any mesh-type or porous material may be used, such as afabric of some sort, or a metal screen or a porous plastic sheet. Nylonnetting is an example of a fabric that may be used and a cooper oraluminum screen is an example of a metal screen that may be used. Inusing the backing material, the first step is that of forming or givingit the desired shape. This is not a problem where a fairly rigidmaterial is used such as the wire screen or porous plastic mentionedabove, but it would be a problem where a fabric or cloth material isused. Accordingly, in the case where a fabric or similar type ofmaterial is employed, it is necessary to first sufficiently rigidize thematerial so that it will hold its shape. Toward this end, the fabric maybe painted or sprayed with or immersed in a substance that will rigidizeit and thereby allow it to hold the configuration into which it has beenshaped. Wax or paraffin is an example of one substance that may be usedalthough many are available. Next, after shaping the backing structurematerial, it is essential to make sure that the material is electricallyconducting. Here again, no problem is presented in the case of a metalscreen, but the use of a. fabric or plastic material does present aproblem. Hence, in the event that any one of the latter materials isused, it is necessary to coat it with an electrically conductivesubstance such as chemical silver which, as was previously mentioned,can be sprayed on.

With the backing structure affixed to the nickel layer thus fardeposited, the entire submaster structure is rcturned to theelectroplating bath until the nickel layer has attained its desiredthickness. A novel feature of this further electroplating of the nickellayer is that the electroplating occurs through the mesh backingstructure. Stated differently, after being returned to the bath, nickelis deposited on both the backing structure and the nickel layer, theflow of nickel ions and their deposition on nickel layer 17 occurringthrough the openings or pores in the backing structure. Thus, as hasalready been mentioned, the final nickel layer and backing structure aswell as the permanent connection or mounting between the two are formedsimultaneously. In other words, in one step backing structure 18 becomesintegrally affixed to nickel layer 17, which constitutes an improvementover the method as it was previously practiced.

At this point, the entire structure is removed from the bath, washed,cleaned and dried, and the plastic subma-ster structure separated fromthe mirror structure. The mirror structure that separates from thesubmaster structure is shown in FIGS. 6 and 6(a) and, as shown therein,includes silver layer 16 which adhered to and separated with nickellayer 17. The separation may be effected by means of pneumatic,hydraulic, mechanical or other means, the techniques for providing suchseparations being well known. in the arts and practiced with greatfinesse.

The next step is that of chemically stripping the silver layer from thenickel layer and to do so in such a manner that the optical surface ofthe mirror, which is the common surface between the silver and nickellayers, is not degraded. To achieve this end, a solution composed ofammonium hydroxide (NH OH), hydrogen peroxide (H and distilled water canbe used to strip the silver. from the nickel without etching the nickel.Assuming'that this has now been done, the final electroformed mirrorproduct comprising nickel layer 17 and its rigidizing or supportstructure 18 is shown in FIG. 7. It should be mentioned, however, thatif deemed necessary, a hard scratch and abrasion resistant surface maybe provided over the nickel face to protect it and such materials asrhodium or chromium may be plated on as the protective film.

Although a particular structure and process have been illustrated anddescribed above by way of example, it is not intended that the inventionbe limited thereto. ihus, for example, the submaster may be made inother ways than as herein described, such as by electroforming it, andused in the same manner to produce the final product. In this regard, itmust be emphasized once again that the present method for supplying abacking structure may be adapted and applied to the electroforming ofdevices other than mirrors. Accordingly, the invention should beconsidered to include any and all modifications, alterations orequivalent arrangements falling within the scope of the annexed claims.

Having thus described the invention, what is claimed is:

1. A reflecting mirror comprising: a layer of metal electroformed as thereflecting surface of the mirror; and a rigid mesh backing structuremounted on said metal layer at least substantially on the edge thereoffor the firm and undistored support thereof, said backing structureincluding a pliable mesh material and an electroformed film of saidmetal that covers said pliable mesh material to rigidize it, said metalfilm blending with said metal layer as a continuation thereof to firmlyand ermanently mount said backing structure as an integral part of saidreflecting surface layer.

2. An apparatus comprising: a thin layer of metal electroformed into apredetermined shape; and a rigid mesh backing structure mounted on saidmetal layer at least substantially on the edge thereof for the firm andundistorted support thereof, said backing structure including a pliablemesh material and an electroformed film of said metal that coats saidmaterial to rigidize it, said metal film blending with said metal layeras a continuation thereof to firmly and permanently mount said backingstructure as an integral part of said metal layer.

3. An apparatus comprising: a thin layer of metal electroformed into apredetermined shape; and a rigid mesh backing structure mounted on saidmetal layer at least substantially on the edge thereof for the firm andundistorted support thereof, said backing structure including a pliableelectrically non-conducting mesh material, a thin coating of asensitizing substance over said mesh material to render it electricallyconducting, and an electroformed film of said metal that covers saidcoating to rigidize said mesh material, said metal film blending withsaid metal layer as a continuation thereof to firmly and permanentlymount said backing structure as an integral part of said metal layer.

4. A reflecting mirror comprising: a layer of metal electroformed as thereflecting surface of the mirror; and a rigid mesh backing structuremounted on said metal layer along the edge thereof and spaced from ittherebetween, said backing structure including a pliable electricallynon-conducting mesh material, a thin coating of a sensitizing substanceover said mesh material to render it electrically conducting, and anelectroformed film of said metal that covers said coating to rigidizesaid mesh material, said metal film blending with said metal layer as acontinuation thereof to firmly and permanently mount said backingstructure as an integral part of said metal layer.

5. A method of fabrication of an electroformed device, said methodcomprising the steps of: forming a mold having the desired shape of thedevice; electroplating a first portion of a metal layer over the workingsurface of said mold; affixing a mesh backing structure onto said firstportion of said metal layer at least substantially on the edge thereof;further electroplating a second portion of said metal layer onto saidbacking structure and through the openings in said backing structureonto said first portion of metal; and separating said mold from thedevice.

6. A method of fabricating an electroformed reflecting mirror, saidmethod comprising the steps of: forming a mold whose working surface hasthe desired shape of the mirrors reflecting surface; electroplating afirst layer of metal over the working surface of said mold; positioningan appropriately-shaped mesh backing structure over said first layer sothat the backing structure is in contact with the metal layer all alongits edge; affixing said mesh backing structure to said first layerwhereat the two are in contact; electroplating a second layer of metalover both said first metal layer and said backing structure; andseparating said mold from the mirror.

7. A method of fabricating an electroformed reflecting mirror, saidmethod comprising the steps of: forming a mold whose working surface hasthe desired shape of the mirrors reflecting surface; sensitizing theworking surface of said mold by coating it with a film ofelectrically-conductive material; mounting said mold as the cathode inan electroplating bath and keeping it there until a first layer of metalis deposited over the sensitized working surface of the mold; removingsaid mold from the bath and positioning an appropriately-shaped meshbacking structure over said first layer so that the backing structure isin contact with the metal layer all along its edge; applying a substanceall along the line of contact between the backing structure and thefirst layer of metal to cause them to adhere to each other; immersingsaid cathode in said bath until a second layer of metal is electroplatedover both said first metal layer and said backing structure; andseparating said mold from the mirror.

References Cited UNITED STATES PATENTS 770,835 10/1903 Tubman 3502931,872,221 8/1932 Bart 2047 2,352,923 7/1944 Turner 350310 XR 3,091,5785/1963 Hetherington 20416 HOWARD S. WILLIAMS, Primary Examiner.

W. VAN SISE, Assistant Examiner.

1. A REFLECTING MIRROR COMPRISING: A LAYER OF METAL ELECTROFORMED AS THEREFLECTING SURFACE OF THE MIRROR; AND A RIGID MESH BACKING STRUCTUREMOUNTED ON SAID METAL LAYER AT LEAST SUBSTANTIALLY ON THE EDGE THEREOFFOR THE FIRM AND UNDISTORED SUPPORT THEREOF, SAID BACKING STRUCTUREINCLUDING A PLIABLE MESH MATERIAL AND AN ELECTROFORMED FILM OF SAIDMETAL THAT COVERS SAID PLIABLE MESH MATERIAL TO RIGIDIZE IT, SAID METALFILM BLENDING WITH SAID METAL LAYER AS A CONTINUATION THEREOF TO FIRMLYAND PERMANENTLY MOUNT SAID BACKING STRUCTURE AS AN INTEGRAL PART OF SAIDREFLECTING SURFACE LAYER.